The present invention relates to a semiconductor device, and more particularly, to a resin-mold type semiconductor device packaged with an epoxy-based resin. The present invention is also concerned with a manufacturing method for the resin-mold type semiconductor device.
A conventionally-used resin-mold type semiconductor device is usually manufactured as follows: after a semiconductor wafer is prepared in a wafer processing step, it is diced into semiconductor chips. The semiconductor chip thus prepared is subsequently mounted on a leadframe (called "die bonding step"). Then, electrode pads formed on the semiconductor chip are connected to inner leads of the leadframe with a thin Au or Al wire (called "wire bonding step"). Finally, the semiconductor chip, the accompanying thin wires, and the inner leads are sealed with a resin (called "packaging step").
Now, the conventional resin-mold type semiconductor device will be explained with reference to FIG. 1, which is a cross-sectional view of one of the resin-mold type semiconductor devices, namely, TSOP (Thin Small Outline Package) type semiconductor device.
Semiconductor chip 1 is fixed on a die pad 6 of a leadframe 4 with a bond 7. The terminal electrodes 2 for power supply and input/output signals formed on the surface of the semiconductor chip 1 are electrically connected to inner leads 8 of the leadframe 4 by the corresponding bonding wires 3 such as a thin Au metal wire. The semiconductor chip 1, bonding wires 3, die pad 6, and inner leads 8, except for the outer leads 5, are sealed with a resin to form a resin-mold package 9. Thereafter, the outer leads 5 are cut in a desired form and bent. In this manner, finished devices are obtained.
Sealing is generally performed after the semiconductor chip is mounted on the leadframe which is formed of inner leads, outer leads, and a die pad. The surface of the semiconductor chip 1, except for electrode 2 formation portions, is protected with an insulating passivation film 10 (such as a silicon oxide film).
Examples of the sealing resins generally used are an epoxy resin mixed with a silica filler, a phenol resin, a cresol resin, a mixture of powder carbon and silicon resin particles, and the like.
The epoxy-based resin is characterized in that it has a high hygroscopicity and a high water permeability. When the epoxy series resin is used to form the resin-mold package 9, external moisture is absorbed or permeates into the package and concentrates in interfaces between heterogeneous materials, for example, between the semiconductor chip 1 and the resin-mold package 9, and between the die pad 6 and the resin-mold package 9.
Incidentally, when a surface mounting type semiconductor device is used, the resin-mold package is bonded on a wiring board with a solder by heating it to a high temperature in a so-called solder reflow processing. In the solder reflow processing, heat (240.degree. C.) is applied to the upper surface of the wiring board or the package is placed in an atmosphere of 215.degree. C. for several seconds. In short, the entire package is exposed to heat ranging from 215 to 240.degree. C.
Such abrupt heating causes rapid vaporization and expansion of the moisture which has concentrated in the interface between heterogeneous materials in the semiconductor device. As a result, bulge and exfoliation occur in the resin-mold package and cracks are also generated in the interior portion. The most serious problem is the exfoliation of the resin-mold package from the die-pad.
The cracks generated in the resin-mold package allow external water and contaminants to enter, with the result that metal corrosion occurs in the terminal electrodes 2 for power supply and input/output signals formed on the surface of the semiconductor chip 1. The external contaminants include Na, K, Cl, and the like which are contained in a soldering flux and a cleaning solvent. If exfoliation takes place in the semiconductor chip 1 and cracks are generated on the upper portion of the semiconductor device, the thin metal wire 3 will be broken, causing electrical breakage.
The cracks and wire breakage will not take place in the resin-mold package if the external water does not enter and remain in the resin-mold package. To prevent external water from entering, coating of the surface of the resin-mold package with a waterproof film is known as a useful technique.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2-280363 discloses a technique for coating the surface of the resin-mold package with a resin having a high moisture-resistance and heat-resistance. This is a technique for coating the surface of the resin-mold package with a silicon resin (coefficient of water absorbency: 0.05%, glass transition temperature: 300.degree. C.) or a polyphenylene sulfide resin (coefficient of water absorbency: 0.05%, thermal deformation temperature: 300.degree. C.), in a thickness of 5 .mu.m or more.
Since the moisture resistance of the resin coating is high, the external water can be almost completely prevented from entering. This means conversely that internal water hardly goes out. Accordingly, the resin-mold package coated with the resin expands more than a non-coated one when extremely high heat is applied in a short time, for example, at the reflow processing. As a result, the resin-mold package with the resin coating is easily broken.
Therefore, to attain the resin-mold type semiconductor device, it is important to prevent the presence of the external and internal moisture in the resin-mold package at the time of heat processing.